1. Field of the Invention
The present invention generally relates to the manufacturing of optical fibers, and particularly to processes for forming glass preforms for the optical fibers. Specifically, the invention concerns a process and an apparatus for the elongation of an optical fiber preform.
2. Description of the Related Art
Several processes are known for making glass preforms for drawing optical fibers. Such processes include the modified chemical vapour deposition (MCVD) process, the outside vapour deposition (OVD) process and the vapour axial deposition (VAD) process.
Many of the known processes for making the preforms include a stage, called elongation, in which a vitrified preform rod, formed according to different techniques depending on the specific manufacturing process, is submitted to reduction in diameter to obtain a preform of prescribed final diameter. To this purpose, the vitrified preform rod is heated, in a furnace or by means of a burner, up to the softening temperature. The preform rod is then stretched so as to reduce the diameter thereof in the softened region, referred to as “neck”. The vitrified preform rod may have a central hole that, during the elongation stage, may collapse.
Several solutions are known for stretching the preform. According to some solutions, both ends of the preform are moved, while the heated region of the preform is kept steady. In this case, the preform is generally elongated along a vertical axis (“vertical elongation process”), and it is attached at the upper end thereof to a feeder; the feeder sustains the preform and feeds it to a furnace. At the bottom end, downstream the furnace, the preform is attached to a puller, providing the traction force necessary to stretch the preform.
Within the furnace, the preform is heated up to the softening temperature. The puller imparts a translation speed higher than the feed rate of the feeder, thereby the softened region of the preform is stretched. The outer diameter of the preform in the softened region is thus reduced and, if present, the preform central hole may collapse. Optionally, the feeder and the puller may also impart to the preform a rotation about its axis.
According to other solutions, one end of the preform is kept steady and the other end is moved, while the heated region is moved in a direction same as or opposite to the moving end of the preform. In this solutions, the preform is generally elongated along a horizontal axis (“horizontal elongation process”). The preform is heated by means of a horizontally movable heater, e.g. a burner mounted on a carriage. The preform ends are attached to mandrels of a horizontal lathe: one mandrel is kept steady, while the other is moved horizontally. The translation speeds of the movable mandrel and the heater determine the final diameter of the preform. Also in this case, the preform may be rotated about its axis.
Irrespective of the specific solution adopted for stretching the preform, the main objective of the preform elongation stage is that of obtaining rods of a prescribed diameter, to be submitted to subsequent processing up to the drawing of optical fibers. It is therefore of paramount importance to monitor the preform diameter during the elongation stage.
Various techniques have been proposed for monitoring the preform diameter during the elongation stage. Generally speaking, all these techniques call for measuring the preform diameter in a limited number of discrete points (one, two or three points) along the preform axis, particularly along the neck, for example by means of laser-based instruments; the measured diameter or diameters are typically compared to predetermined diameter values, and the feed rate of the feeder and/or the speed of the puller, or the speed of the movable mandrel and/or the speed of the heater, depending on the solution adopted for stretching the preform, are controlled accordingly. For example, assuming that the measured diameter is higher than the target diameter, the speed of the movable mandrel is increased, and vice versa.
Techniques providing for measuring the preform diameter in one prescribed point along the preform axis are for example described in JP 57092534, JP 62108743, JP 61014149, U.S. Pat. No. 5,755,849 and U.S. Pat. No. 5,942,019. JP 5147971, U.S. Pat. No. 6,178,778 and JP 8091861 are examples of prior art documents describing the measurement of the preform diameter in two or three discrete points along the preform axis.
In particular, U.S. Pat. No. 5,942,019, in relation to the elongation of preforms by means of a furnace, underlines the importance of setting the position for measuring the outer diameter of the taper portion (i.e., the neck). Summarising, in that document it is observed that in the case where the outer diameter measuring position is disposed near the upper end of the taper portion, i.e., near the heater, even when the moving speeds of the chucks are controlled to keep the outer diameter of the upper end of the taper portion constant, the outer diameter may be varied at the taper portion, thereby the outer diameter of the elongated body may become uneven and fluctuate. On the other hand, in the case where the outer diameter measuring position is disposed near the lower end of the taper portion, since the glass preform has almost been cooled at this position and its viscosity has been quite large to be elongated, even if a fluctuation in the outer diameter is detected, it can hardly be corrected. Still according to U.S. Pat. No. 5,942,019, the optimal outer diameter measuring position, which varies depending on the outer diameter of the glass preform before elongating, the outer diameter of the elongated body, the heater temperature, the inner diameter of the furnace core tube and the like, is to be determined experimentally.
According to the Applicant, determining the optimal position of the diameter measurement point by means of experiments is not satisfactory from an industrial application viewpoint.
The Applicant has moreover noticed that prior art methods provides for measuring the diameter in one, two or three prefixed points, in particular in points that have no correlation with the actual geometry of the neck of the preform being elongated, and that because of the dependency of the neck length and shape on several process parameters, such as the initial and final diameter of the preform, the process operating speeds, the temperature profiles, the speed of rotation of the preform, if provided, the diameter of the preform central hole, if present, and the internal pressure, the neck geometry may vary from process to process and even in the course of a single elongation process. The Applicant has found that, for this reason, measuring the neck diameter in prefixed points, which are not correlated to the geometry of the neck, does not allow a precise control of the preform final diameter.